Superior white cast iron



Nov. 1, 1966 w. H. MOORE SUPERIOR WHITE CAST IRON Filed June 11. 1962 FIG. 2

United States Patent 3,282,683 SUPERIOR WHITE CAST IRON William H. Moore, Larchmont, N.Y., assignor to Meehanite Metal Corporation, a corporation of Missouri Filed June 11, 1962, Ser. No. 201,704 2 Claims. (Cl. 75123) My invention relates to a white cast iron and, more specifically to a process for producing white cast iron that has a preferred carbide structure resulting in greater toughness and improved wear resistance.

White iron castings are castings where the excess carbon occurs as iron carbides which exhibit a white fracture when the casting is broken. Carbides are noted for both hardness and brittleness and it is well known to those skilled in the art that the amount and the form of the free carbide in the casting will determine its exact behavior in use.

While both the amount of the free carbide and its hardness may be adjusted by varying the composition of the cast iron, particularly in the direction of changing the carbon content or using carbide-forming elements such as chromium, vanadium, bismuth, magnesium, tellurium, sulphur, and manganese, etc. it has long been realized that this type of control does not completely determine the exact properties and behavior of the end product.

It is an object of this invention to provide an improved method of carbide formation and control in a white cast iron.

Another object of the invention is to produce a white iron having improved mechanical properties, particularly toughness.

Another object of the invention is to produce a white iron having a higher proportion of free carbides that exhibit a plate-like structure, rather than a eutectic type structure.

Another object of the invention is to provide a means of control that will enable the production of white iron of a consistent quality in any casting section.

Other objects and a fuller understanding of my invention may be had by referring to the following description and claims:

In a white cast iron two basic types of carbide may occur viz., the eutectic type or ledeburite appearing as in FIGURE 1 and the undcrcooled or plate-type appearing as in FIGURE 2. The eutectic type of FIGURE 1 is by far the most common and is the essential component of all white cast irons. Being massive in size and occuring close together, these carbides provide a path for easy fracture, which leads to inherent brittleness in the cast iron in which they occur.

The undercooled, or plate-type, carbide, on the other hand, being less massive and presenting a greater area of matrix between particles of carbides, leads to an inherently tougher type of cast iron. toughness usually results in better wearing characteristics, because it is well known that wear resistance is a function of both hardness and toughness and with some types of wear, i.e., wear under pounding impact, the toughness of the metal is by far the more important factor.

When a white cast iron is made strongly white, in other words, it possesses a degree of carbide stability that would give carbides in even heavier or slower-cooling casting sections, the tendency is to form more massive carbides, with consequent decrease of toughness. ods of control favored by those skilled in the art is to produce a cast iron which does not have excess carbide stabilization in relation to the section of the casting. This is done by adjusting the composition or by addition of some graphitizing agent, so that the iron will solidify This enhanced One of the methice white in the section of the casting, but if poured into a heavier casting section, it would not be completely white. This type of control involves the use of fracture tests such as wedge tests and visual observation of the fracture in relationship to the casting to be poured.

A common test of this type is the wedge test where the width of the wedge at the line of demarcation between the white fracture and gray fracture forms a measure of the section of casting that would be just white. Thus, a wedge measuring inch across the line of demarcation would represent a'metal which, if poured into a casting of /2 section, would be just white. It the wedge value were less than this, then some free graphite would be expected to occur in a /2 section.- If, on the other hand, the wedge value was greater than this, then the carbides occurring in the /2" section would be more massive and less desirable in structure.

It has been known to melt an iron which would have a Wedge value somewhat greater than the section to be poured and then reduce this wedge value close to the desired value by the addition of a suitable graphitizer. Thus, a white iron could be produced by using a final wedge value from 1 to 1% times white as related to the casting section. For example, on a 1 section the wedge value would be to in value.

The conditions for the production of a greater quantity of plate-type carbides in a white iron are not completely known, but it has been postulated that such plate carbides result from a condition of undercooling in the metal. It follows, therefore, that conventional control techniques, such as the use of graphitizers which prevent undercoole ing, will also effectively prevent the formation of the more desirable plate-type carbide in the structure. My invention is based on a unique method of control and ladle addition which I have found will greatly increase the proportion of undcrcooled or plate carbides in the structure of the resultant white iron.

I have found that under the right conditions a late ladle addition of a carbide stabilizing or a carbide metastabilizing addition will produce a high proportion of undercooled or plate carbides in the structure. By a late addition is meant an addition after melting but before casting. This addition is preferably the last-one made before casting.

The correct conditions for a suitable late addition are an initial degree of carbide instability in the melt. If a melt is produced which would have a small quantity of graphite in the structure, i.e. the wedge is less than one times white and a late addition of a carbide former is then made so that the graphite is eliminated from the structure and the wedge is greater than one times white, then a large proportion of plate-type carbides will occur in the structure.

The first step of the process of this invention involves producing a melt which, if cast into a given section, would show a slight proportion (up to about 25%) of free graphite in the structure. This gives the well known mottled fracture, which, in terms of wedge fracture tests, would fall in the range of to 1 times white. In a 1" casting section this corresponds to a wedge value of to 7 This melt may be produced with a variety of compositions and would embrace those quantities of carbon, silicon, manganese, and other elements commonly found in white or semi-white cast irons. I

The second step of this invention involves the late addition to the molten metal of carbide stabilizing or metastabilizing elements such as bismuth, chromium, manganese, molybdenum, tungsten, vanadium, uranium, columbium, tellurium, boron, tantalum or metastabilizers of the nodular impelling type such as lithium, calcium, magnesium,

sulphur, cerium, lanthanum, neodymium, praseodymium,

yttrium in such amount that the wedge value of the metal is increased, so that there is no free graphite in the structure. In terms of fracture or wedge values, this would correspond to a 1 to 1% times white, or a to wedge value for a 1" casting section.

I have found that the particular carbide stabilizing agent used is not vital, in that all carbide stabilizers under these conditions will produce the plate-type of carbides. However, I have found that the metastabilizing agents, such as bismuth, cerium, or magnesium, are particularly effective and appear to produce a greater proportion of undercooled or plate-type carbides in the final metal.

These agents should be added in suificient quantity to overcome all neutralizing effects and produce increased whiteness in the melt. Thus, for example, sulphur neutralizing elements such as lithium, cerium, or magnesium, must be added in suflicient quantity to first neutralize sulphur and then provide an increase in carbide stability. By the same token, elements such as sulphur, must be added in sufficient quantity to first neutralize manganese before any whitening action could be expected to occur,

As far as carbide stabilizing elements are concerned, chromium, and molybdenum appear to be the most effective in producing a high proportion of plate type carbides.

The amount added will vary according to conditions and while a small addition will often produce some plate type carbides, a larger addition will usually produce more of these carbides, providing the final wedge value of the metal is not greatly higher than that required by the casting section. In the case of chromium for example, I have used additions of as low as .25% and as high as 1.5%, depending on the wedge value before treatment and also on the Wedge value after treatment.

It appears to be preferable to limit the degree of whitening by the late addition to the melt of carbide stabilizing or metastabilizing elements. Thus, a very heavy late addition will tend to produce more stable carbides of the eutectic type, whereas a more moderate addition will allow sufficient undercooling to produce a greater proportion of the plate-type of carbide. For this reason, the preferred embodiment of this invention contemplates starting with a mottled iron and then making it just white by means of a ladle addition of carbide-forming material.

The process of the invention is best illustrated by example.

A melt was produced having the following composition:

Percent Total carbon 3.20 Silicon 1.28

Manganese 1.46 Sulphur 0.06 Phosphorus 0.10 Chromium 0.34

A portion of the melt was cast into a 1.2" diameter test bar and into a wedge test casting of a wedge with a 2" back and a 28 /2 angle face. A second portion was cast into a 1.2" diameter bar and wedge made in carbon sand for accelerated cooling. To a third portion of this melt a ladle addition of ferro-chrome was made, so as to increase the chromium content of the melt by 0.60%. This treated portion was also cast intoa 1.2 diameter test bar and a wedge with a 2" back.

To a fourth portion of this melt a ladle addition of 0.25% of bismuth was made and this treated portion was cast into a 1.2 diameter test bar and a wedge with a 2" back.

The wedge test pieces were fractured and measured at the line of demarcation, whereas the 1.2" test bars were broken in an impact testing machine delivering 250 foot pounds of striking energy. All broken bars were subsequently polished and examined under the microscope.

The results of these tests are tabulated in Table 1.

The base metal portion exhibited some free graphite and reduced carbide content, thus accounting for its relatively high impact strength. The same metal chilled more rapidly so as to produce fu-ll whiteness, had a reduced impact strength. Both portions of the melt treated in accordance with the process of this invention produced a higher impact strength and a greater proportion of the plate-type carbides.

I have found that castings made according to the teaching of this invention exhibit a high degree of toughness and excellent wear resistant qualities in conditions of service involving abrasion. These improved characteristics appear to be due to the higher proportion of plate-type carbides in the structure.

This invention has been described with a certain degree of particularity. in its preferred form, but it should be understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of construction and operation may be made, without departing from the spirit and scope of the invention hereinafter claimed.

What is claimed is:

1. A tough wear resistant white cast iron article of manufacture having a greater proportion of plate type carbides and a lesser proportion of eutectic type carbides produced by providing an initial melt having a first carbide value of less than 1 times white and free graphite in the structure not exceeding about 25%, adding a carbide forming element to the melt in sufficient amount to provide a second carbide value in the range of from 1 to 1% times white with substantially no free graphite in the structure, and thereafter casting the melt.

2. A tough wear resistantwhite cast iron article of manufacture having a greater proportion of plate type carbides and a lesser proportion of eutectic type carbides produced by providing an initial melt having a first carbide value of less than 1 times white and free graphite in the structure not exceeding about 25 adding a carbide forming element to the melt selected from the group consisting of bismuth, chromium, manganese, molybdenum, tungsten, vanadium, uranium, columbium, tellurium, boron, tantalum, lithium, calcium, magnesium, sulphur, cerium, lanthanum, neodymium, praseodymium, yttrium, in sufficient amount to provide a second carbide value in the range of from 1 to 1% times white with substantially no free graphite in the structure and thereafter casting the melt.

References Cited by the Examiner UNITED STATES PATENTS 2,253,502 8/1941 Boegehold 75-123 2,370,225 2/ 1945 Boegehold 75123 2,450,395 9/1948 Eckman et a1. 75-123 2,579,452 12/1951 Eckman et al 75-123 FOREIGN PATENTS 106,743 2/ 1939 Australia. 147,610 7/ 1952 Australia.

DAVID L. RECK, Primary Examiner.

HYLAND BIZOT, Examiner.

H. W. TARRING, Assistant Examiner. 

1. A TOUGH WEAR RESISTANT WHITE CAST IRON ARTICLE OF MANUFACTURE HAVING A GREATER PROPORTION OF PLATE TYPE CARBIDES AND A LESSER PROPORTION OF EUTECTIC TYPE CARBIDES PRODUCED BY PROVIDING AN INTITAL MELT HAVING A FIRST CARBIDE VALUE OF LESS THAN 1 TIMES WHITE AND FREE GRAPHITE IN THE STRUCTURE NOT EXCEEDING ABOUT 25%, ADDING A CARBIDE FORMING ELEMENT TO THE MELT IN SUFFICIENT AMOUNT TO PROVIDE A SECOND CARBIDE VALUE IN THE RANGE OF FROM 1 TO 1 1/4 TIMES WHITE WITH SUBSTANTIALLY NO FREE GRAPHITE IN THE STRUCTURE, AND THEREAFTER CASTING THE MELT. 